Biofuel is a promising renewable energy technology in part because of the large amount and low cost of its biomass feedstock. Efficient action of cellulases to release fermentable sugars from biomass cellulose is an important step in making this conversion economically viable. The major strategies to improve cellulase activity include rational design and directed evolution. Rational design is based on knowledge of the structure of cellulases, and presumes a detailed understanding of the relationship between enzyme structure and its function, but directed evolution does not require understanding of structure and function.
Clostridium thermocellum is an anaerobic, thermophilic, cellulolytic, and ethanogenic bacterium that shows potential for use in bioenergy production because it is capable of directly converting cellulose into ethanol. Degradation of cellulosic materials by Clostridium thermocellum is carried out by a large extracellular cellulase system called the cellulosome, a complicated protein complex consisting of nearly 20 different catalytic subunits. One feature of the cellulosome is the nonhydrolytic scaffoldin subunit that integrates the various catalytic subunits into the complex via interactions between its repetitive cohesin domains and complementary dockerin domains on the catalytic subunits. Several cellulolytic bacteria and fungi are known to produce extracellular multienzyme complexes similar to the cellulosome.
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